The disclosure of Japanese Patent Application No. 2002-133457 filed on May 9, 2002, including the specification, drawings, and abstract is incorporated herein by reference in its entirety.
1. Field of the Invention
The invention relates to control of exhaust gas recirculation in an internal combustion engine.
2. Description of the Related Art
In internal combustion engines typified by gasoline engines, various arts are utilized to reduce amounts of emission substances and improve fuel consumption. Variable valve timing is one of such arts. Variable valve timing is an art in which overlap period in which both intake and exhaust valves of an internal combustion engine are open is changed by adjusting timings for opening and closing the intake valves. In general, an increase in overlap period makes it possible to reduce amounts of emission substances and improve fuel consumption. This is grounded on the fact that combustion gas is burned again after having been temporarily blown back into an intake pipe and sucked again into cylinders, and the fact that HC contained in combustion gas are burnt again at this moment (this phenomenon is referred to as internal EGR). Variable valve timing is utilized for the purpose of enhancing volumetric efficiency of intake air and increasing output as well as reducing amounts of emission substances and improving fuel consumption.
External EGR is another art of reducing amounts of emission substances and improving fuel consumption. External EGR is an art in which part of exhaust gas of an internal combustion engine is recirculated to the intake side. HC contained in combustion gas are burnt again by carrying out external EGR, whereby it becomes possible to reduce amounts of emission properties and improve fuel consumption.
The temperature of exhaust gas to be recirculated in the case of external EGR is lower than the temperature of exhaust gas to be recirculated in the case of internal EGR, and thus makes it possible to lower temperatures in cylinders. Therefore, external EGR has an effect of inhibiting the occurrence of knocking during high-load operation. On the contrary, internal EGR makes it possible to inhibit temperatures in the cylinders from dropping. Therefore, internal EGR has an effect of stabilizing combustion even during low-load operation. From the standpoint described above, there is proposed an art in which external EGR and internal EGR are selectively utilized such that external EGR is carried out only in a range of high loads and that internal EGR is carried out in ranges of lower loads, according to control regulations that are determined in advance by engine speed and engine load (e.g., as disclosed in Japanese Patent Application Laid-Open No. 4-175449).
By performing control such that internal EGR and external EGR are selectively carried out according to the control regulations determined in advance, however, it is impossible to achieve with certainty a desired effect of improving fuel consumption. Namely, the originally intended effect of improving fuel consumption is not achieved in some cases, depending on environmental factors relevant to operation of the internal combustion engine, temperatures of the internal combustion engine, and the like. Further, detailed studies on control during a transition between internal EGR and external EGR have not been conducted yet. In some cases, therefore, torque fluctuations are caused during a transition between internal EGR and external EGR.
The invention has been made to solve the aforementioned problem. It is an object of the invention to achieve with certainty a reduction in amounts of emission substances and an improvement in fuel consumption by selectively carrying out two kinds of exhaust gas recirculation, that is, internal EGR and external EGR in an internal combustion engine. It is another object of the invention to suppress torque fluctuations during a transition between internal EGR and external EGR.
In order to solve at least part of the aforementioned problem, an operation of an internal combustion engine is controlled in a manner described below according to a first arrangement of the invention. The internal combustion engine to be controlled includes a variable valve timing mechanism (hereinafter referred to as xe2x80x9cVVTxe2x80x9d) capable of adjusting overlap period during which both intake and exhaust valves are open, and a recirculation mechanism for recirculating part of exhaust gas to the intake side.
A control device detects operational state of the internal combustion engine, and controls internal EGR amount based on overlap and external EGR amount based on the recirculation mechanism on the basis of control regulations that are set in advance. Operational state of the internal combustion engine can be detected, for example, using parameters including engine speed, engine load, and the like. An increase in overlap makes it possible to increase internal EGR amount. By enhancing time density (hereinafter referred to as xe2x80x9copen dutyxe2x80x9d) in which the recirculation mechanism is open, it becomes possible to increase external EGR amount. Furthermore, if the occurrence of knocking is forecast or detected during this control, the control device modifies the control regulations so as to avoid knocking. In forecasting the occurrence of knocking, it is sufficient that a high possibility of the occurrence of knocking be forecast. Even if the occurrence of knocking is forecast, continuation of operation under a corresponding condition may not always cause the actual occurrence of knocking.
In a second arrangement of the invention, if the occurrence of knocking is detected, a period of delay may be reduced or eliminated. This makes it possible to control the VVT swiftly and to avoid knocking swiftly. In general, since knocking tends to occur when overlap amount is large, it is preferable to control the VVT so as to reduce overlap amount.